Fuel cells are cells that directly convert the chemical energy generated by oxidation of a fuel to electrical energy, and attention is being paid to the fuel cells as a next-generation energy source due to their environment-friendly features of high energy conversion efficiency and reduced contaminant discharge.
A fuel cell generally has a structure in which an anode and a cathode are formed, with an electrolyte membrane interposed therebetween, and such a structure is called a membrane-electrode assembly (MEA).
Fuel cells can be classified into alkaline electrolyte fuel cells, polymer electrolyte membrane fuel cells (PEMFC), and the like, and among them, polymer electrolyte membrane fuel cells are attracting more attention as power source apparatuses for portable, automobile and domestic applications, due to their advantages such as a low operation temperature such as below 100° C., fast starting and fast response characteristics, and excellent durability.
A representative example of such a polymer electrolyte membrane fuel cell is a proton exchange membrane fuel cell (PEMFC) that uses hydrogen gas as the fuel.
To briefly describe the reaction occurring in a polymer electrolyte membrane fuel cell, first, when a fuel such as hydrogen gas is supplied to an anode, an oxidation reaction of hydrogen occurs at the anode, and thereby hydrogen ions (H+) and electrons (e−) are produced. The hydrogen ions (H+) thus produced are transferred to a cathode through a polymer electrolyte membrane, and electrons (e−) thus produced are transferred to the cathode through an external circuit. Oxygen is supplied to the cathode, and oxygen binds with hydrogen ions (H+) and electrons (e−), and water is produced by a reduction reaction of oxygen.
Since the polymer electrolyte membrane is a channel through which the hydrogen ions (H+) produced at the cathode are transferred to the cathode, the polymer electrolyte membrane should essentially have excellent conductivity for hydrogen ions (H+). Furthermore, the polymer electrolyte membrane should have excellent separation capability of separating hydrogen gas that is supplied to the anode and oxygen that is supplied to the cathode, and should also have excellent mechanical strength, dimensional stability, chemical resistance and the like, and characteristics such as a small ohmic loss at a high current density are required.
One of those polymer electrolyte membranes that are currently in use may be an electrolyte membrane made of a perfluorosulfonic acid resin as a fluororesin (hereinafter, referred to as “fluorine ion conductor”). However, a fluorine ion conductor has weak mechanical strength, and thus has a problem that when used for a long time, pinholes are generated, and thereby the energy conversion efficiency is decreased. In order to increase the mechanical strength, there has been an attempt of using a fluorine ion conductor having an increased membrane thickness; however, in this case, there is a problem that the ohmic loss is increased, and the use of expensive materials increases, so that the economic efficiency is low.
In order to address such problems, there has been suggested a polymer electrolyte membrane having enhanced mechanical strength by impregnating a porous polytetrafluoroethylene resin (trade name: TEFLON) (hereinafter, referred to as “Teflon resin”), which is a fluororesin, with a liquid-state fluorine ion conductor. In this case, the hydrogen ion conductivity may be somehow inferior as compared with polymer electrolyte membranes composed of a fluorine ion conductor alone; however, the impregnated polymer electrolyte membrane is advantageous in that the polymer electrolyte membrane has relatively superior mechanical strength, and can therefore have a reduced thickness, so that the ohmic loss is decreased.
However, since a Teflon resin has very poor adhesiveness, there are limitations on the selection of the ion conductor, and in the case of products produced by applying fluorine ion conductors, the products have a disadvantage that the fuel crossover phenomenon occurs conspicuously as compared with hydrocarbon-based ion conductors. Furthermore, because not only fluorine ion conductors but also porous Teflon resins are highly expensive, there still is a demand for the development of a new inexpensive material for the mass production of fuel cells.